This research will investigate the structure and acquisition of American Sign Language (ASL), a natural language of the deaf of North America. ASL may be acquired as a primary language either early or late in life, and either from native signers or from signers who themselves acquired the language late in life. Its study therefore provides an unusual opportunity to investigate the consequences of early vs. late experience, and of wide variations in input environment, on the process by which a language is acquired and the ultimate character of its users' knowledge. Eight experiments are proposed, to test adult users of ASL as well as children who are in the process of acquiring ASL, with a test battery of ASL phonology, morphology, and syntax which we will devise. Given our previous results, these experiments are expected to demonstrate: 1) that there are striking differences in the knowledge of a language that users attain as a function of when they began to learn that language, and 2) that there is striking uniformity in the knowledge of a language that users attain when they begin learning the language in infancy, despite wide variations in input environments. In addition, the experiments should reveal whether these phenomena are due to the nature of the acquisition process, to hitherto unnoticed details of the input, or to interference (or the lack of interference) from English. The results should contribute to our understanding of the importance of early experience for language acquisition, and to the character of learning in childhood vs. adulthood. In addition, they should contribute to decisions regarding language exposure, whether spoken or signed, in deaf education and parent counseling.